One way in which the performance of a communication link is monitored is by monitoring a bit error rate (BER) of the link. A BER is typically defined as the number of errors that occur during the transmission or reception of a given number of bits. The transmission or reception of a bit is referred to here as “processing” a bit. The errors include, for example, cyclic code redundancy (CRC) errors or other code violations.
In some applications, a threshold BER value is established so that when the BER exceeds the threshold BER, some action takes place. For example, an alarm can be raised, the rate at which data is processed can be altered, or alternate or additional communication media can be used. For example, in one embodiment of a high-speed digital subscriber line 4 (HDSL4) system, an HDSL4 BER (HBER) threshold for an HDSL4 communication link used in such a system is defined as 1 error in one million bits (expressed as “1E-6”). Therefore, if data is transmitted at the rate of 1.544 million bits per second (that is, at T1 speed), the HBER threshold corresponds to an HBER of 1.544 million bits per second times 1 error per million bits times 60 seconds, or 92.64 errors in each 60-second period. An HBER threshold of 1E-6 corresponds, therefore, to an HBER of approximately 93 errors in a 60-second period.
One way in which a BER reading is obtained is by counting, for a given period of time, the number of errors that occur. Then, the BER for that period is calculated by dividing the number of errors counted by the number of bits processed in a given period. The number of bits processed in a given period can be obtained by counting each processed bit or by calculating the number of processed bits based on the line speed. It is common to average several such BER readings in an attempt to improve the accuracy of the calculated BER. The average is then, for example, compared to a BER threshold.
In some communication systems, BER readings tend to vary widely. For example, BER readings in some devices include erroneous values that are very large relative to other, accurate readings. This variation can result from the equipment used to make the BER reading and/or from the environment in which the equipment is used. As a result, when several BER readings are averaged, one large, erroneous reading can cause the resulting average to be greater than a specified BER threshold regardless of what the other measurements are. In some situations, this can increase the number of false BER alarms that are generated.